This invention relates to electric motors and, more particularly, to a system and method for improving the transient speed response of electric motors in the presence of drive current response rate limitations.
Drive systems for variable speed electric motors are often limited in how quickly they can change the speed of the electric motor. This is due to the fact that the rate of change in the drive current needed to effectuate a rapid change in motor speed cannot be achieved due to motor design limitations and/or the lack of an adequate forcing voltage.
U.S. Pat. No. 4,280,081, which is herein incorporated by reference in its entirety, discloses an inertia compensation method to improve the acceleration and deceleration performance of motor drive systems. However, this method, as well as other prior art inertia compensation methods, do not address the limited transient speed response imposed by drive current response rate limitations (hereinafter xe2x80x9ccurrent rate limitationsxe2x80x9d).
In an exemplary embodiment of the invention, a drive system for an electric motor comprises a current reference generator that generates and outputs a current reference based on a speed reference, a current rate limiter that receives the current reference and outputs an adjusted current reference based on available current, a current rate recovery system that determines total accumulated lost current-seconds due to the current rate limiter and outputs current-seconds to be added to the adjusted current reference, and an adder that adds the current-seconds to the adjusted current reference output by the current rate limiter until the total accumulated lost current-seconds are applied to the electric motor.
The invention also provides a current rate recovery system for an electric motor drive system, comprising a comparator that determines a difference between a current reference and an adjusted current reference generated by the electric motor drive system, and an integrator that receives and integrates the difference between the current reference and the adjusted current reference over time, and outputs at least a portion of the integrated difference between the current reference and the adjusted current reference as current-seconds to be added to the adjusted current reference.
The present invention also provides a method of compensating for current-rate limitations in an electric motor drive system, comprising the steps of monitoring total accumulated current-seconds lost due to a lack of available current, and recovering the total accumulated lost current-seconds as sufficient current becomes available.
The present invention keeps track of the total accumulated lost current-seconds due to current rate limitations, and restores the total accumulated lost current-seconds to the system when current becomes available.